The principal goal of this project is to develop uses of images derived from electron cryomicroscopy as molecular replacement models for x-ray crystallography. A second goal is to use high-resolution structures of viral proteins and subviral particles, together with cryoEM reconstructions of intact virions, to obtain detailed models for complex viruses. (1) Human papillomavirus 16 (HPV16) small virus-like particles (VLPs) will be used as test objects, to work out the best ways of determining the orientation and position parameters for placing a 3-D image reconstruction in the unit cell of a crystal and to analyze the resulting phase correlations in different resolution shells. We will analyze the effects of CTF corrections, procedures for deriving the reconstructed image, and molecular- replacement algorithms on the phase-determining power of the model. (2) Molecular replacement approaches will be used to solve the phase problem for rotavirus inner capsid particles (ICPs), for the rotavirus outer-shell protein, VP7, and for the retrovirus" penetration complex", mu13SIGMA3/3. 93) Crystal structures for components of retrovirus and rotavirus particles will be used, together with EM reconstructions of the intact viruses, to build models for the intact virions.